Operation of lean burn engines, for example, diesel engines and lean burn gasoline engines, provide the user with excellent fuel economy and have low emissions of gas phase hydrocarbons and carbon monoxide due to their operation at high air/fuel ratios under fuel lean conditions. Additionally, diesel engines offer significant advantages over gasoline (spark ignition) engines in terms of their fuel economy, durability, and their ability to generate high torque at low speed.
From the standpoint of emissions, however, diesel engines present problems more severe than their spark-ignition counterparts. Because diesel engine exhaust gas is a heterogenous mixture, emission problems relate to particulate matter (PM), nitrogen oxides (NOx), unburned hydrocarbons (HC), and carbon monoxide (CO).
NOx is a term used to describe various chemical species of nitrogen oxides, including nitrogen monoxide (NO) and nitrogen dioxide (NO2), among others. NO is of concern because it is believed to undergo a process known as photo-chemical smog formation, through a series of reactions in the presence of sunlight and hydrocarbons, and is a significant contributor to acid rain. NO2, on the other hand, has a high potential as an oxidant and is a strong lung irritant.
Effective abatement of NOx from lean burn engines is difficult to achieve because high NOx conversion rates typically require reductant-rich conditions. Conversion of the NOx component of exhaust streams to innocuous components generally requires specialized NOx abatement strategies for operation under fuel lean conditions.
One such strategy for the abatement of NOx in the exhaust stream from lean burn engines uses NOx storage reduction (NSR) catalysts, which are also known as “lean NOx trap (LNT).” The lean NOx trap technology can involve the catalytic oxidation of NO to NO2 by catalytic metal components effective for such oxidation, such as precious metals. However, in the lean NOx trap, the formation of NO2 is followed by the formation of a nitrate when the NO2 is adsorbed onto the catalyst surface. The NO2 is thus “trapped”, i.e., stored, on the catalyst surface in the nitrate form and subsequently decomposed by periodically operating the system under fuel-rich combustion conditions that effect a reduction of the released NOx (nitrate) to N2.
Oxidation catalysts comprising a precious metal dispersed on a refractory metal oxide support are known for use in treating the exhaust of diesel engines to convert both hydrocarbon and carbon monoxide gaseous pollutants by catalyzing the oxidation of these pollutants to carbon dioxide and water. Such catalysts have been generally contained in units called diesel oxidation catalysts (DOC), which are placed in the exhaust flow path from a diesel-powered engine to treat the exhaust before it vents to the atmosphere. Typically, the diesel oxidation catalysts are formed on ceramic or metallic substrate carriers (such as, e.g. a flow-through monolith carrier), upon which one or more catalyst coating compositions are deposited. In addition to the conversions of gaseous HC, CO, and the soluble organic fraction (SOF) of particulate matter, oxidation catalysts that contain platinum group metals (which are typically dispersed on a refractory oxide support) promote the oxidation of nitric oxide (NO) to NO2.
An alternative strategy for the abatement of NOx under development for mobile applications (including treating exhaust from lean burn engines) uses selective catalytic reduction (SCR) catalyst technology. The strategy has proven effective as applied to stationary sources, e.g., treatment of flue gases. In this strategy, NOx is reduced with a reductant, e.g., NH3, hydrocarbon, or urea-based reagents, to nitrogen (N2) over an SCR catalyst that is typically composed of base metals. This technology is capable of NOx reduction greater than 90%, thus it represents one of the best approaches for achieving aggressive NOx reduction goals.
Particulate matter (PM) is a concern with respect to diesel emissions, as particulates are also connected to respiratory problems. To address both the threat to human health and the need for greater fuel efficiency that diesel engines provide, governmental regulations have been enacted curbing the amount of particulate matter allowed to be emitted from a diesel engine. The two major components of particulate matter are the volatile organic fraction (VOF) and a soot fraction (soot). The VOF condenses on the soot in layers, and is derived from the diesel fuel and oil. The VOF can exist in diesel exhaust either as a vapor or as an aerosol (fine droplets of liquid condensate), depending upon the temperature of the exhaust gas. Soot is predominantly composed of particles of carbon. The particulate matter from diesel exhaust is highly respirable due to its fine particulate size, which poses health risks at higher exposure levels. Moreover, the VOF contains polycyclic aromatic hydrocarbons, some of which are suspected carcinogens.
To satisfy government regulations regarding particulate matter emissions, soot filters have been used. Catalyzed soot filters (CSF), also called diesel particulate filters, are designed to reduce emission of particulate matter from diesel engines. The filters first trap particulates and then use catalytic technology to continuously burn them at normal diesel operating temperatures. When using a CSF, the filter must be periodically regenerated by burning off the particulate matter. However, because the temperature where particulate matter ignites is significantly higher than the normal operating temperature of a diesel engine, catalysts have been used to reduce the ignition temperature of the particulate matter.
The catalysts on the CSF can enhance the oxidation of the particulate matter. Generally, catalysts for CSFs contain alkali or alkaline oxides to reduce the particulate matter ignition temperature. However, these catalysts are often volatile and/or destructive to the filters, resulting in impractically short lifetimes. Additionally, these catalysts require a substantial amount of noble metal catalysts to reduce the HC and CO gases that are emitted along with the particulate matter.
Other oxides, such as rare earth oxides and base metal oxides have also been used in conjunction with noble metal catalysts to attempt to lower the particulate matter ignition temperature while also catalyzing the HC and CO emissions. These catalysts, however, tend to require substantial amounts of noble metal catalysts and/or rare earth oxides. Thus, making these catalysts very expensive to produce.
Unfortunately, as engine modifications are made to diesel engines in order to reduce particulates and unburned hydrocarbons, the NOx emissions tend to increase.
In a DOC+CSF+SCR catalyst system, both DOC and CSF can make NO2. However, the NO oxidation function on the DOC is less important, and the NO2/NOx ratio before the SCR catalyst is controlled by the CSF. Therefore, in such a system, in addition to its soot function (filtration and regeneration), the CSF is also an NO oxidation catalyst, improving the NOx conversion over the SCR catalyst.
Urea injection is an important element for the SCR technology, particularly reliable and precise control of the injection. Because SCR efficiency is related to NO2/NOx ratio, for a given NOx conversion target, urea injection quantity depends on both NOx level and NO2/NOx ratio. From a control stand point, it is much more desirable to have a constant NO2/NOx ratio (varying within a narrow range) at the inlet of the SCR catalyst. That requires the NO oxidation activity on the CSF catalyst to be very stable. Existing DOC and CSF catalyst and systems have not been able to meet the NO oxidation stability requirement.
Therefore, there is an ongoing need to develop a catalyzed honeycomb substtrate that provides a stable ratio of NO2/NOx for downstream components such as the SCR catalyst.